Communication method

ABSTRACT

The invention relates to a method for cyclical communication between communications stations ( 1, 2, 3 ) which are provided for controlling or surveying a technical process ( 50 ), through a bus ( 4 ). According to the invention, communications relations ( 12, 13, 21, 31 ) which are provided for the communication stations ( 1, 2, 3 ) are executed during each bus cycle of predetermined duration (Δt) and if a communications relation is disturbed, its repetition ( 21 W) is scheduled for a succeeding bus cycle and the disturbed communications relation is acknowledged with a special acknowledgement code.

[0001] The invention relates to a method for cyclic communicationbetween communication stations provided for controlling or monitoring atechnical process, via a bus, in which communication sessions plannedfor the communication stations are executed during in each case one buscycle.

[0002] Such a communication method is known from the standard EN 50 170or PROFIBUS standard. The PROFIBUS is a so-called field bus which isused for communicatively linking communication stations provided forautomating a technical process. A communication station at this or asimilar bus is, e.g., a so-called stored-program control (SPC). Afurther communication station at the bus is, e.g., a so-calleddecentralized peripheral device to which external sensors or actuatorscan be connected for controlling or monitoring the technical process.

[0003] Controlling a technical process frequently also includesclosed-loop control tasks. In this context, closed-loop controlcomprises picking up a measurement value from the technical process andoutputting control information to the technical process. Both thepicking-up of the measurement value and the outputting of the controlinformation is usually cyclic. Since the measurement value is thusavailable not continuously but only in each case at the time when it ispicked up, i.e. at the sampling time, this is a sampled-data controlsystem, the quality or stability of which is primarily dependent on theequidistance of the sampling times.

[0004] Frequently, the measurement value is picked up from the processby a first communication station and processed by a second communicationstation. Using the measurement value, this second communication stationalso generates the control information to be output. The controlinformation is then output to the process by a third communicationstation. The distance between two sampling times, and, therefore, thesampling frequency is thus determined by the duration of thecommunication between the respective communication stations.

[0005] To ensure equidistance of the sampling times, a constant buscycle time is provided, e.g. in the PROFIBUS. The bus cycle time is thetime interval within which all cyclic communication sessions planned forthe communication stations connected to the bus are executed exactlyonce. A communication method with constant bus cycle time is known,e.g., from German patent application 199 39 182 (date of application20.08.1999).

[0006] A communication comprises the transfer of a message via the busfrom the transmitting communication station to the receivingcommunication station. The time required for transferring a message isessentially determined by the volume of data transferred. The volume ofdata of cyclic communications, however, is essentially constant. Thus,an approximate equidistance between the individual communications isobtained with a constant bus cycle time. The approximate equidistance ofthe individual communications is accompanied by an approximateequidistance of the sampling times because the measurement value pickedup from the process is a data item of a communication or, respectively,a message.

[0007] To ensure actual equidistance, the bus cycle time is longer thanthe time which would be required for executing all plannedcommunications. The additional time is available as reserve for messageretransmissions and so-called acyclic messages. If no messageretransmissions are required in a bus cycle or there are no acyclicmessages ready for transfer, the system still waits until thepredetermined bus cycle time (including the spare time) has elapsedbefore it begins the next bus cycle. This results in a fixed timingpattern for the planned communications, by means of which theequidistance of the samples can be ensured.

[0008] The disadvantageous factor in this known communication method is,however, that this equidistance is no longer guaranteed in the case of adisturbed communication session. According to EN 50 170, in the case ofa disturbed communication session, this session is repeated between once(1 time) and fifteen times (15 times) in the same bus cycle. This leadsto the duration of the bus cycle being extended by the durationresulting from the repetition of the disturbed communication session.Equidistance of individual communication sessions over a number of buscycles—as is required, in particular, for critical sampled-data controlsystems—cannot be guaranteed with the known communication method in thecase of communication and/or transmission disturbances.

[0009] The reason for this is that each communication session which isexecuted after the disturbed communication session in time in the buscycle is offset in time in comparison with a “normal” bus cycle, i.e. abus cycle without disturbed communication session. This also lowers thequality of a sampled-data control system. In the extreme case, even thestability of the sampled-data control system can be put in question.

[0010] The invention is based on the object, therefore, of specifying acommunication method by means of which equidistant sampling of ameasurement value of the technical process is also still possible in thecase of transmission disturbances.

[0011] According to the invention, this object is achieved by means ofthe features of claim 1. Advantageous further developments andembodiments are the subject matter of the subclaims.

[0012] For this purpose, it is provided in a method for cycliccommunication between communication stations provided for controlling ormonitoring a technical process, via a bus, in which communicationsessions planned for the communication stations are executed during ineach case one bus cycle of predeterminable duration, that, in the caseof a disturbed communication session, its retransmission is planned fora subsequent bus cycle and that the disturbed communication session isacknowledged with a special acknowledgement code. According to analternative, no retransmission of the disturbed communication sessiontakes place after a disturbed communication session which isacknowledged with a special acknowledgement code. This can be toleratedsince, in the case of a cyclic communication, a retransmission isreplaced by the communication session of the next cycle. In thisarrangement, it is possible to adjust whether no retransmission at all,one, two etc. retransmissions of the disturbed communication session areto take place.

[0013] The advantages achieved by means of the invention consist, inparticular, in that the duration of the bus cycle with the disturbedcommunication session remains unaffected by retransmission and/orcorrection measures. This is achieved by displacing a retransmission ofa disturbed communication session into a subsequent bus cycle. In thecase of an immediate retransmission of a disturbed communication sessionin the same bus cycle, the starting time of all communication sessionsfollowing the disturbed communication session in the bus cycle isdisplaced with reference to the bus cycle. Equidistance of thesedisplaced communication sessions over a number of bus cycles would nolonger be guaranteed.

[0014] If values which are included in a sampled-data control system aretransmitted by means of the displaced communication sessions, thesampling frequency of the sampled-data control systems concerned is nolonger constant which is accompanied by a deterioration in the qualityof the sampled-data control system and possibly even instability of thesampled-data control system. By displacing the retransmission of adisturbed communication session into a subsequent bus cycle, thestarting times of the subsequent communications essentially remainunaffected. Thus, the equidistance of the individual communicationsessions is also guaranteed over a number of bus cycles so that thecommunication method is also suitable for critical sampled-data controlsystems.

[0015] So that no retransmission and/or correction measures take placein the bus cycle with the disturbed communication sessions, thedisturbed communication session is acknowledged with a specialacknowledgement code. This leads to the omission of, in particular,transmission and/or correction measures so that the remainingcommunication sessions planned for the bus cycle can be executed.

[0016] If further disturbed communication sessions occur in the same buscycle, the remaining disturbed communication sessions are also dealtwith like the first disturbed communication session. i.e. itsretransmission, too, is displaced into a subsequent bus cycle.

[0017] So that the retransmission of a disturbed communication sessionin a subsequent bus cycle does not impair the equidistance of thecommunication sessions planned for this subsequent bus cycle, theretransmission of the disturbed communication session takes placefollowing the communication sessions planned for this bus cycle. Due tothe fact that the retransmission of the disturbed communication sessiontakes place following the communication sessions planned for this buscycle, their starting times, based on the bus cycle, remain unaffected.The equidistance of the communication sessions over a number of buscycles is thus guaranteed even during the retransmission of thedisturbed communication session.

[0018] The retransmission of the disturbed communication session isadvantageously planned for a bus cycle immediately following the buscycle with the disturbed communication session. As a result, the timedifference between the disturbed communication session, the time of theplanned execution and the retransmission, the time of the actualexecution, remains as small as possible.

[0019] If the disturbed communication session is acknowledged with thespecial acknowledgement code like a faultless communication session, adelay in the execution of the communication sessions following in thebus cycle is avoided. It is only in the case of a communication sessionwhich has been executed faultlessly or acknowledged as faultless thatthe execution of the communication sessions following in the bus cyclecan be continued. Due to the acknowledgement of the disturbedcommunication session with the special acknowledgement code, thissession cannot be distinguished from a faultless communication session.First, the beginning of the execution of the communication sessionsfollowing in the bus cycle is not delayed by any evaluation of a specialacknowledgement code.

[0020] The special acknowledgement code is advantageously converted intoa normal acknowledgement code. This reduces the number of possibleacknowledgement codes to be checked so that, in spite of the existenceof an acknowledgement code which is additional in fact, the complexityof the analysis of the possible acknowledgement codes is not increased.The normal acknowledgement code is the acknowledgement code whichidentifies a communication session which has been faultlessly executed.The special acknowledgement code can be converted into a normalacknowledgement code, e.g. by means of so-called “masking”. If, e.g.,the special acknowledgement code only differs from the normalacknowledgement code due to the first bit being set, the masking canbegin by a logical AND operation on the special acknowledgement code,e.g. with the hexadecimal value “7FFF”.

[0021] If a retransmission counter is provided by means of which thenumber of retransmissions of the disturbed communication session iscounted, it can be determined at any time how long the disturbance ofthe communication session already exists.

[0022] If a limit value is provided for the retransmission counter andany retransmission of the disturbed communication session isacknowledged with a fault acknowledgement code after the limit value hasbeen reached, a disturbance of a communication session which lasts “toolong” can be detected.

[0023] The limit value from which each retransmission of the disturbedcommunication session is acknowledged with a fault acknowledgement codecan be suitably predetermined. Thus, the time interval after the passageof which a disturbance of a communication session exists for “too long”can be predetermined. The limit value can be preferably predeterminedindividually for each communication session. Thus, the limit value for acommunication session which, e.g., supplies data for a critical firstsampled-data control system can be set to a different limit value thanthat of a communication session by means of which only record data aretransmitted.

[0024] If the retransmission counter can be read out by thecommunication station involved in the disturbed communication session,it can determine at any time whether and possibly for how long acommunication session is disturbed.

[0025] If at least one threshold value is provided for theretransmission counter and planned measures are initiated by thecommunication station reading out the retransmission counter when thethreshold value is reached, it is also possible to respond to thedisturbance of the communication session in a suitable manner evenbefore the limit value is reached. For example, when a first thresholdvalue is reached, a corresponding note can be output, for instance on ascreen or printer, when a second threshold value is reached a visual ororal warning can be initiated and when a third threshold value isreached, fault-limiting measures can be initiated. The fault-limitingmeasures can consist, e.g., in attempting to establish the communicationsession on a different path—for example with a redundant bus-, orplacing the technical process—possibly the part process controlled bythe communication station—into a safe state.

[0026] Advantageously, a structure is provided in a memory of at leastone communication station which has for each communication session onefield in which the value of the retransmission counter is stored in afirst position, the limit value is stored in a second position and theat least one threshold value, together with a reference to the measureto be initiated when the threshold value is reached, is stored in athird position. The structure is used for the compact storage of theessential data which are provided for executing the communicationmethod. Storing a reference to the measure to be initiated when thethreshold value is reached makes it possible to directly call up aprogram routine in which the measure is programmed.

[0027] In the text which follows, an exemplary embodiment of theinvention will be explained in greater detail with reference to adrawing, in which:

[0028]FIG. 1 shows communication stations communicatively connected viaa bus for controlling a technical process,

[0029]FIGS. 2a and 2 b show communication sessions between individualcommunication stations,

[0030]FIG. 3 shows faultlessly executed communication sessions duringtwo bus cycles,

[0031]FIG. 4 shows a message retransmission as a consequence of adisturbed communication session,

[0032]FIG. 5 shows a message retransmission in a later bus cycle,

[0033]FIG. 6 shows a layout of a data structure, and

[0034]FIG. 7 shows a flowchart.

[0035]FIG. 1 shows communication stations 1, 2, 3, the communicationstation designated by the reference symbol 1 being a so-called master1—e.g. a stored-program control—and the communication stationsdesignated by the reference symbols 2, 3 being so-called slaves 2,3—e.g. decentralized peripheral devices. The communication stations 1,2, 3 are communicatively connected to one another via a bus 4.

[0036] The master 1 is a communication station which has an activetransmit authorization on the bus 4. A slave 2, 3, in contrast, onlytransmits after having first been addressed by the master 1. Slaves 2, 3are, therefore, lacking the active transmit authorization because theyonly respond to a request (being addressed) by the master 1.

[0037] The communication stations 1, 2, 3 are provided for controllingor monitoring a technical process 50 shown diagrammatically. Thetechnical process 50 comprises a reactor 51 with an inlet 52 and anoutlet 53. The reactor 51 is fed by the inlet 52. A reagent 54 leavesthe reactor 51 via the outlet 53. The inlet 52 is controlled by a valve55. A filling level meter 56 is used for determining a filling level 54′of the reactor 51.

[0038] A simple control and/or monitoring (automation) of the technicalprocess 50, which will be used by way of an example in the text whichfollows, can consist in that the valve 55 is controlled with a view to aconstant filling level 54′ of the reactor 51.

[0039]FIGS. 2a and 2 b show the data exchange or, respectively,communication sessions 12, 21, 13, 31 between communication stations 1,2, 3 for this automation. The data exchange takes place under control ofa program 6, stored in a memory 5, which is executed by the master 1.For this purpose, the program 6 comprises a task 7, 7′ which is executedin a fixed timing pattern and is called up, e.g., every 500 ms. Witheach execution of the task 7, 7′, a data exchange 12, 21 and 13, 31,respectively, takes place between the master 1 and the slave 2 (FIG. 2a)and between master 1 and slave 3 (FIG. 2b).

[0040] In accordance with a planned communication session, a dataexchange takes place between the relevant communication stations 1, 2,3. The data exchange takes place by means of a message 12, 21, 13, 31.In the text which follows, therefore, the termscommunication/communication session and message are used synonymously.If the master 1 addresses a slave 2, 3, this takes place by means of amessage 12, 13. The slave 2, 3, in turn, responds to this stimulus witha message 21, 31.

[0041]FIG. 2a shows a message 12 sent by the master 1 to the slave 2.The message 12 comprises e.g. output and control data in the form ofdigital and/or analog values, e.g. a maximum value for the filling level54′. However, the message 12 also causes the slave 2 to send its inputdata to the master 1 in a message 21. The message 21 thus contains, inparticular, a value representing the filling level 54′ of the reactor 51which is picked up by means of the filling level meter 56. The message12 implicitly issues to the slave 2, which is not actively authorized totransmit, an authorization for transferring the data requested by themaster 1. This is done by means of the message 21.

[0042]FIG. 2b shows a message 13 by means of which the master 1 sends tothe slave 3 the respective output data. The message 13 comprises outputand/or control data in the form of digital or analog values, among themalso a value predetermining the position of the valve 55. The message 13causes the slave 3 to send its input data, e.g. also a valuerepresenting the actual volume flowing through the inlet 12, to themaster 1 in a message 31.

[0043] A message 12, 13, 21, 31 is always acknowledged 12′, 13′, 21′,31′. The faultless undisturbed transmission of a message 12, 13, 21, 31is acknowledged by means of a normal acknowledgement which specifiesthat the communication 12, 13, 21, 31 has been executed faultlessly. Thenormal acknowledgement comprises, e.g., a normal acknowledgement code(not shown) with the value “0x00”.

[0044] Automation of the technical process 50 (FIG. 1) requires aclosed-loop control for keeping the filling level 54′ of the reactor 51constant. Since the message 21 comprises the measurement value of thefilling level 54′ and the latter is thus “sampled” only when the slave 2transmits the corresponding message 21 to the master 1, this is asampled-data control system. The quality/stability of such asampled-data control system is mainly determined by the time intervalbetween two samples of a process parameter (in this case filling level54′). To achieve stable closed-loop control, the basic mathematicalcontrol models require sampling at equidistant times.

[0045]FIG. 3 diagrammatically shows the times t1_(n), t2_(n), t1_(n+1),t2_(n+) ₁ cluttered along a time axis, at which the messages 21, 13 aretransmitted to and from the corresponding slave 2, 3.

[0046] The slave 2 transmits the current filling level 54′ of thereactor 51 to the master 1 with the message 21. At times t1_(n),t1_(n+1), the process parameter “filling level” 54′ is thus sampled. Thenew position of valve 55 is predetermined by master 1 for slave 3 withmessage 13. Messages 12 (FIG. 2a) and 31 (FIG. 2b) are not shown forreasons of clarity.

[0047] To obtain stable control of the filling level 54′, it isimportant that the time interval between two successive times t1_(n),t1_(n+1), at which the filling level 54′ is sampled, remains constant.The time difference between time t1_(n) and t2_(n+1), i.e. the pickingup of the filling level 14′ and the outputting of the resultant controlvalue to the process 50, represents a dead time which can be easilytaken into consideration and compensated for mathematically.

[0048] In the case of an undisturbed faultless communication, theequidistance between two successive times t1_(n), t1_(n+1) is guaranteedby the fixed timing pattern in which the task 7, 7′ (FIGS. 2a, 2 b) isexecuted under the control of which the communication sessions areexecuted.

[0049] The time interval Δt designates the duration of a bus cycle, theterms bus cycle and duration of a bus cycle being used synonymously inthe text which follows. The duration of a bus cycle (bus cycle time) Δtis constant. During a bus cycle Δt, all planned communication sessionsare executed. If the starting time of task 7, 7′ (FIGS. 2a, 2 b), underthe control of which the data exchange 21, 13 between the master 1 andthe slaves 2, 3 is executed, falls into a bus cycle Δt, thecommunication sessions 21, 13 belong to the communication sessionsplanned for this bus cycle Δt.

[0050]FIG. 3 shows two bus cycles Δt which in each case comprise thecommunication sessions 21, 13. These two bus cycles Δt do not follow oneanother directly in time—indicated by the broken timeline. Between thetwo bus cycles shown, one or more other bus cycles are executed which donot include the communication sessions 21, 13.

[0051]FIG. 4 shows the effect of a disturbed communication session 21^(s) on the duration of a bus cycle Δt. In the second bus cycle Δt_(s)shown, a disturbance which impairs the execution of the communicationsession 21 occurs at time t1_(n+1). The disturbed communication session21 ^(s) is acknowledged with a fault acknowledgement 21″. Eachacknowledgement 21′, 21″ comprises an acknowledgement code 21′, 21″ sothat this fault acknowledgement 21″, too, comprises a faultacknowledgement code 21″ which unambiguously species the type of thefault. The disturbed communication relation 21 ^(s) is followed by aretransmission 21 ^(w) of the disturbed communication session. The firstretransmission 21 ^(w) of the disturbed communication session cannot beexecuted faultlessly, either, and is, therefore, acknowledged with afault acknowledgement 21″. It is only after the second retransmission 21^(w) that it can be faultlessly executed. The faultlessly executedsecond retransmission 21 ^(w) is correspondingly acknowledged with anormal acknowledgement 21′. The normal acknowledgement 21′ comprises anormal acknowledgement code 21′ which specifies the faultless execution.

[0052] Overall, the communication session 13 is thus executedcorrespondingly later in time, namely only at time t2′_(n+1). In thecase of a bus cycle Δt (FIG. 3) which is not encumbered by a disturbedcommunication session, in contrast, the execution occurs at timet2_(n+1). The time offset by which the communication session 13 isexecuted later corresponds to the duration of the two retransmissions 21^(w) of the disturbed communication session 21 ^(s).

[0053] The new value for the position of the valve 55 (FIG. 1) istransmitted with the message 13. The intervention in the closed-loopcontrol is thus delayed, i.e. no longer at equidistant times so that itmay no longer be possible to keep the filling level 54′ (FIG. 1)constant. The more dynamic the controlled system the stronger this willaffect the quality of the closed-loop control. In the extreme case, eventhe stability of the closed-loop control can be put in question.Furthermore, the duration Δt_(s) of the bus cycle with the disturbedcommunication 21 ^(s) is extended in comparison with the duration Δt ofthe bus cycle with the faultlessly executed communication 21. This leadsto communication sessions executed in each bus cycle also no longerbeing equidistant.

[0054]FIG. 5 shows how equidistance is guaranteed even in the case of adisturbed communication session 21 ^(s). It shows two bus cycles Δt_(s),Δt immediately following one another. In the first bus cycle Δt_(s), adisturbance 21^(s) occurs. A third bus cycle Δt is executed later intime—indicated by the broken timeline.

[0055] Analogously, a disturbance 21^(s) occurs during the execution ofthe communication session 21—at time t1_(n+1). The disturbedcommunication session 21 ^(s) is acknowledged with a specialacknowledgement 21′ which comprises a special acknowledgement code 21′like a faultlessly executed communication session. I.e., the specialacknowledgement code 21′ is converted into a normal acknowledgement code21′ or the special acknowledgement code 21′ is evaluated like a normalacknowledgement code 21′. Thus, there is no immediate retransmission ofthe disturbed communication session 21′ in the same bus cycle Δt_(s). Onthe contrary, the retransmission of the disturbed communication session21 ^(s) is planned for the next bus cycle Δt.

[0056] Thus, there is no time offset in the transmission of the newvalue for the position of the valve 55 by means of the message 13 evenin the bus cycle Δt_(s) with the disturbed communication session 21^(s). The message 13 is still executed at time t2_(n+1) as before.

[0057] The retransmission 21 ^(w) of the disturbed communication session21 ^(s) is executed at time t1_(n+1) ^(w) in the bus cycle Δt followingimmediately. For such a retransmission 21 ^(w), a special communicationsection t30 is provided at the end of each bus cycle Δt. Plannedcommunication sessions 13, 21 are executed at the beginning of each buscycle Δt in a normal communication section t20.

[0058] The number of retransmissions 21 ^(w) of a disturbedcommunication session 21 ^(s) is counted in a retransmission counter 111according to FIG. 6. If the retransmission counter 111 reaches apredetermined limit value 112, each retransmission 21 ^(w) of thedisturbed communication session 21 ^(s) is acknowledged with a faultacknowledgement (not shown) which comprises a fault acknowledgementcode, when the limit value 112 is reached. Thus, permanently disturbedcommunication sessions can be recognized as such and the communicationstation which can no longer be reached can be flagged as failed.

[0059] Dividing a bus cycle Δt into the normal communication section t20and the special communication section t30 leads to a decoupling betweenmessages 13, 21 of correspondingly planned communication sessions andmessage retransmissions 21 ^(w) due to disturbed communication sessions21 ^(s). The message retransmission 21 ^(w) in bus cycle Δt—i.e. in thesecond bus cycle in FIG. 5—is decoupled from planned communicationsessions (shown dashed) to be executed in the normal communicationsection t20.

[0060] Since the duration of the bus cycle Δt is predetermined andconstant, either the duration of the normal communication section t20 orthe duration of the special communication section t30 is alsopredetermined. If the duration of the normal communication section t20is predetermined, the fixed bus cycle time Δt will produce the durationof the special communication section t30 and vice versa. The duration ofthe normal communication section t20 and special communication sectiont30 is dimensioned in such a manner that at least one messageretransmission 21 ^(w) can take place during the special communicationsection t30.

[0061]FIG. 6 shows a structure 100 which is provided in the memory 5(FIGS. 2a, 2 b) of a communication station 1, 2, 3. The structure 100has a separate field 110, 120, 130 for each communication session—atleast for each communication session in which the relevant communicationstation 1, 2, 3 is involved. In each field 110, 120, 130, the value ofthe retransmission counter 111, 121 is stored in a first position, thelimit value 112, 122 is stored in a section position and the at leastone threshold value 114, 124, together with a reference 115, 125 to themeasure to be initiated when the threshold value 114, 124 is reached, isstored in a third position 113, 123.

[0062] The structure is used for compact storage of the essential datawhich are provided for executing the communication method. Storing areference 115, 125 to the measure to be initiated when the thresholdvalue 114, 124 is reached enables a program routine to be called updirectly in which the measure is programmed.

[0063] The omission points “. . . ” in the structure 100, on the onehand, and, on the other hand, in the field 130 indicate that thestructure can comprise other fields 110, 120, 130, depending on thenumber of communication sessions, and that the field 130, like any otherfields, basically has the same layout as the field 110, 120.

[0064]FIG. 7 shows in a flowchart an algorithm for essential aspects ofthe communication method, which begins in step 1001 if theretransmission 21 ^(w) of the disturbed communication session 21 ^(s)could not be executed faultlessly either. In step 1010, theretransmission counter 11, 121 (FIG. 6)

[0065] is incremented with each retransmission 21 ^(w) of a disturbedcommunication session 21 ^(s) is (FIG. 5).

[0066] In step 1020, a check is made whether the retransmission counter11, 121 has reached the limit value 112, 122 (FIG. 6). If this is so,the system branches to step 1040 and the retransmission 21 ^(w) of thedisturbed communication session is acknowledged with a faultacknowledgement code 21″ (FIG. 4). After execution of step 1040, thealgorithm is ended in step 1002. If it is found in step 1020 that theretransmission counter 11, 121 has not yet reached the limit value 112,122, the algorithm is continued in step 1030.

[0067] In step 1030, the retransmission 21 ^(w) of the disturbedcommunication relation 21 ^(s) is acknowledged with the specialacknowledgement code 21′ (FIG. 5). The acknowledgement of theretransmission 21 ^(w) of the disturbed communication session 21 ^(s)with the special acknowledgement code 21′ has the effect that a failedretransmission 21 ^(w) of the disturbed communication session 21 ^(s) isalso treated like a faultlessly executed communication and a nextretransmission 21 ^(w) is planned for a following bus cycle.

[0068] In step 1050, a check is made whether the retransmission counter111, 121 has reached the threshold value 114, 124 (FIG. 6)—possibly oneof a number of threshold values. If this is so, the system branches tostep 1060 and a measure is triggered, the reference (address) of whichis stored at position 115, 125 (FIG. 6). If, e.g., the limit value 112,122 is set to the value “20”, the threshold value 114, 124 can be sete.g. to value “10”. I.e. after ten unsuccessful retransmissions 21 ^(w)of a disturbed communication session 21 ^(s), the threshold value 114,124 is reached and a corresponding measure 115, 125 can be initiated.This measure 115, 125 can consist, e.g., of outputting a warning messageon a display device (not shown) in order to indicate the disturbedcommunication session 21 ^(s). The actual measure is implemented asprogram routine (subroutine).

[0069] As a reference 115, 125, its start address is stored at position115, 125. When the threshold value is reached, the measure 115, 125 canbe triggered directly on the basis of the stored reference 115, 125.After step 1060 has been executed, the algorithm is ended in step 1002.If it is found in step 1050 that the retransmission counter 111, 121 hasnot yet reached the threshold value 114, 124, the algorithm is endedimmediately in step 1002.

[0070] The algorithm is started every time in step 1001 even if theretransmission 21 ^(w) of the disturbed communication session 21 ^(s)could not be executed faultlessly. If, in contrast, the firstretransmission 21 ^(w) of the disturbed communication session 21 ^(s)has already been faultlessly executed, this is acknowledged with thenormal acknowledgement code 21′ analogously to step 1030. In this case,evaluation of the limit or threshold value 111, 121 and 114, 124 is notrequired.

[0071] Thus, a method for cyclic communication between communicationstations 1, 2, 3, provided for controlling or monitoring a technicalprocess 50, via a bus 4, is specified in which communication sessions12, 13, 21, 31, which have been planned for the communication stations1, 2, 3, are executed during in each case one bus cycle ofpredeterminable duration Δt. In the case of a disturbed communicationsession, its retransmission 21 ^(w) is planned for a subsequent buscycle and the disturbed communication session is acknowledged with aspecial acknowledgement code.

1. A method for cyclic communication between communication stations (1, 2, 3) provided for controlling or monitoring a technical process (50), via a bus (4), communication sessions (12, 13, 21, 31) planned for the communication stations (1, 2, 3) being executed during in each case one bus cycle of predeterminable duration (Δt), characterized in that, in the case of a disturbed communication session (21 ^(s)), its retransmission (21 ^(w)) is planned for a subsequent bus cycle and in that the disturbed communication session (21 ^(s)) is acknowledged with a special acknowledgement code (21′).
 2. The communication method as claimed in claim 1, in which the retransmission (21 ^(w)) of the disturbed communication session (21 ^(s)) takes place in a subsequent bus cycle following the communication sessions planned for this bus cycle.
 3. The communication method as claimed in claim 1 or 2, in which the retransmission (21 ^(w)) of the disturbed communication session (21 ^(s)) is planned for a bus cycle immediately following the bus cycle with the disturbed communication session (21 ^(s)).
 4. The communication method as claimed in one of claims 1 to 3, in which the disturbed communication session (21 ^(s)) is acknowledged with the special acknowledgement code (21′) like a faultless communication session.
 5. The communication method as claimed in claim 4, in which the special acknowledgement code (21′) is converted into a normal acknowledgement code (21′).
 6. The communication method as claimed in one of claims 1 to 5, in which the number of retransmissions of the disturbed communication session (21 ^(s)) is counted by means of a retransmission counter (11, 121).
 7. The communication method as claimed in claim 6, in which each retransmission of the disturbed communication session (21 ^(s)) is acknowledged with a fault acknowledgement code (21″) after a limit value (112, 122), which can be predetermined individually for each communication session, for the retransmission counter (111, 121) has been reached.
 8. The communication method as claimed in claim 6 or 7, in which the retransmission counter (111, 121) can be read out at least by the communication station (1, 2, 3) involved in the disturbed communication session (21 ^(s)).
 9. The communication method as claimed in one of claims 6 to 8, in which at least one threshold value is provided (114, 124) for the retransmission counter (111, 121), planned measures being initiated by the communication station (1, 2, 3) reading out the retransmission counter (111, 121) when the threshold value (114, 124) is reached.
 10. The communication method as claimed in claim 9, in which a structure (100) is provided in a memory of at least one communication station (1, 2, 3) which has for each communication session (12, 13, 21, 31) one field (110, 120, 130) in which the value of the retransmission counter (111, 121) is stored in a first position, the limit value (112, 122) is stored in a second position and the at least one threshold value (114, 124), together with a reference (115, 125) to the measure to be initiated when the threshold value is reached, is stored in a third position (113, 123). 